An energy storage system (ESS: Energy Storage System) is provided with a charge/discharge function and is used for improving electric power quality such as stabilization of the electric power supplied by a power grid system and suppression of frequency variations in the power grid system. The energy storage system is also used for reducing the peak usage amounts of consumers. Such an energy storage system is expected to expand a market in the future.
For a use to improve the electric power quality of the system, the energy storage system basically works 24 hours a day, 356 days. Therefore, when the deterioration of the energy storage system is to be evaluated, it is desired to evaluate the deterioration without stopping the function of the energy storage system. A current mainstream is to estimate deterioration based on charge/discharge histories. However, state evaluation in the working of a long period such as 20 years has not yet been realized. Meanwhile, there is a method of carrying out deterioration estimation by monitoring and remote monitoring of the energy storage system; however, this has not yet been realized too. In an energy storage system for a vehicle-mount use or an electricity-distribution-side use, the working energy storage system can be stopped; therefore, precise deterioration evaluation can be carried out by stopping the energy storage system and actually carrying out a charge/discharge test. However, in an energy storage system for the use to improve electric power quality in the power grid system, the working energy storage system cannot be stopped; therefore, this method is not realistic.